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Why NOT NULL does not mean valid
A populated field can still contain a blank string, a sentinel value, a value outside an allowed range, malformed text, or a value that conflicts with another field. For example, a product price of zero is present, but may violate a rule that prices must be positive.
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Validity depends on the rule you need to enforce. PostgreSQL describes CHECK as a way to require a row-level expression to hold, while also noting that a CHECK is satisfied when its result is true or null. MySQL 8.4 likewise documents acceptance of TRUE or UNKNOWN for CHECK expressions. Pair CHECK with NOT NULL when a field must be both present and valid: PostgreSQL 18 constraints and MySQL 8.4 CHECK constraints.
Query for rows that violate the rule
Translate the business requirement into a predicate, then select rows for which it fails. These illustrative patterns are not tested against a particular schema or SQL dialect; adjust expressions and types for your database.
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Values outside a range
SELECT *
FROM products
WHERE price <= 0;This finds nonpositive prices if the rule is that price must be greater than zero. If price can also be NULL and NULL is independently invalid, include price IS NULL in the query.
Blank or whitespace-only text
SELECT *
FROM customers
WHERE trim(customer_code) = '';This checks for an empty value after trimming whitespace. Trimming behavior can vary by database, so confirm how your engine handles the characters your data may contain.
Values outside an allowed set
SELECT *
FROM orders
WHERE status NOT IN ('pending', 'paid', 'cancelled');Use the actual allowed values for your application. If NULL is possible and is a separate violation, add status IS NULL.
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Conflicting fields
SELECT *
FROM bookings
WHERE start_date > end_date;This finds date ranges whose start comes after their end. Define whether equal dates are allowed before choosing the comparison operator.
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Review and repair the results safely
- State the rule precisely. Replace vague expectations such as “the value should look right” with a condition you can evaluate, such as “price must be greater than zero.”
- Count and inspect candidates. Count matching rows and review representative records to catch false positives before changing data.
- Confirm the intended correction. A query identifies rows matching your predicate; it cannot establish that the business rule is correct or determine the right replacement value. Get the responsible data owner’s approval before rewriting or deleting production records.
- Repair existing data, then enforce the invariant. Once invalid rows are resolved, add a constraint so future writes cannot violate the rule.
- Check the deployed engine and settings. Confirm the database product and version, whether constraints are enforced, and any relevant input modes before relying on the result.
Choose a constraint that matches the rule
| What must be true | Suitable constraint | Important distinction |
|---|---|---|
| A value must be present | NOT NULL |
Does not rule out blank, malformed, or otherwise invalid non-NULL values. |
| A row must satisfy a predicate | CHECK |
Use for row-level conditions; add NOT NULL separately when NULL is not allowed. |
| A value or combination must be unique | UNIQUE |
Prefer a relational uniqueness constraint over a CHECK that tries to inspect other rows. |
| A reference must point to an existing related row | FOREIGN KEY |
Expresses referential integrity between tables. |
PostgreSQL cautions that CHECK constraints are not a general solution for rules involving other rows: such a check may not maintain lasting consistency as data changes. Use constraints such as UNIQUE or FOREIGN KEY when they express the relationship; PostgreSQL also identifies EXCLUDE constraints for certain exclusion rules. See the PostgreSQL constraints documentation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Verify behavior for your database version
Constraint syntax and enforcement details vary by engine and version. MySQL 8.4 documents CHECK evaluation for INSERT, UPDATE, REPLACE, LOAD DATA, and LOAD XML, as well as differences in behavior for statements using IGNORE: MySQL 8.4 CHECK constraints.
Input handling also matters. The MySQL 8.0 manual says strict SQL mode rejects invalid values by default, while disabling strict mode can allow invalid values to be coerced; it does not recommend relying on that behavior. Verify the configuration actually used by your deployed database: MySQL 8.0 handling of invalid data.
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